§ 5.321 WATER BALANCE 221 



uptake of the fiddler crab, Uca (Scudamore, 1947 and Guyselman, 

 1953); but in the shore crab, Carcinus, as in Camharus, eyestalk 

 removal before moulting results in a volume increase of 180 per 

 cent instead of 80 per cent in the normal crab. Injected sinus gland 

 extracts reduce the former value to 80 per cent and the latter to 

 50 or 60 per cent (CarHsle, 1956). 



At premoult, w^hen the natural secretion of moult-inhibiting 

 hormone begins to fade out, there is known to be an increase in 

 internal osmotic pressure, owing to the mobilization in the blood 

 of materials resorbed from the old skin. In forced moults, following 

 eyestalk ablation, water absorption is greater than in natural 

 moults, although this mobilization proceeds more slowly. The 

 change in internal osmotic pressure in forced moulting cannot 

 therefore be the only cause of increased water uptake. The loss of 

 eyestalks and their hormones must increase the permeability of the 

 tissues to water, as well as inducing the moult and the increase in 

 osmotic pressure. Swelling at the time of natural moulting must 

 then be due to a decrease in the secretion of this hormone, but not 

 to its complete cessation. This would allow the skin, and particu- 

 larly the branchial epithelium, to become more permeable than in 

 the intermoult stage, but not as permeable as in forced moults. 

 Increased permeability of the excretory tissue would also allow of 

 increase in reabsorption of water from the urine, but there seem 

 to be no figures relating to urine flow during moulting. 



Even under natural conditions in sea water, there must be 

 considerable water endosmosis in Carcinus to maintain the high 

 rate of urine flow, amounting to 14 per cent of the blood volume 

 daily. The necessary internal osmotic pressure to achieve this, 

 (unless active transport of water is to be postulated) must be partly 

 due to active inw^ard transport of such ions as Na + , Ca++ and 

 CI" ; but this is small, though it may, perhaps, be aided by the 

 presence in the haemolymph of ionized proteins, which do not 

 pass into the urine. At the same time the endosmosis must be 

 limited by a certain degree of impermeability of the tissues, 

 maintained by the eyestalk hormone, otherwise eyestalk removal 

 would not resuh in increased water uptake. It therefore seems 

 reasonable to postulate that the action of one of the eyestalk 

 hormones is the same as that of other diuretic hormones, namely, 



